The Heat Transfer Characteristics Of Transient Flow Under Rolling Motion Conditions

Experimental studies of low flow rate heat transfer characteristics and temperature fluctuation characteristics under rolling motion were conducted in this paper.The flow loop employed in this experiment was fixed on a rolling device and with water as the test fluid. The testing pipe was made of stainless steel with a cross section of 40×2 mm . The experimental results showed that flow rate fluctuations were induced by the rolling motion, and outlet water temperature fluctuation and outer wall temperature fluctuation were all caused by flow rate fluctuations. The relative flow rate amplitude decreases sharply in laminar region, but the amplitudes of temperature fluctuation are relatively complicated. Because the amplitudes of temperature fluctuation of outlet water and outlet wall decrease initially as the time average Reynolds number increases, then increase,and finally decrease again. The amplitudes of flow rate fluctuation and temperature fluctuation are affected greatly by rolling parameter and Prandtl number. The amplitude of the flow rate fluctuations increases with the increase of the rolling amplitude, rolling frequency and with the decrease of the Prandtl number. Therefore the amplitude of outlet temperature fluctuation and outer wall surface temperature fluctuation increase with the increase of the rolling amplitude, rolling frequency, while decrease with the increase of the Prandtl number.In the transition region, the outlet water temperature fluctuation is not affect evidently by the rolling parameter and Prandtl number. In the turbulent region, the outlet water temperature does not fluctuate very obviously due to the flow rate fluctuates relatively smallThe mechanism of temperature fluctuation amplitude with time average Reynolds number is propounded. The amplitude of temperature fluctuation with time average Reynolds number is affected by both flow rate fluctuation amplitude and heat transfer coefficient. In laminar and turbulent region, the temperature fluctuation amplitude is mainly affected by flow rate fluctuation amplitude because the heat transfer coefficient increases relatively small with time average Reynolds number. In laminar-turbulent transition region, due to the large amplitude of flow rate fluctuation induced by rolling motion, the flow in a complete period goes through laminar and turbulent region, and the heat transfer coefficient increases obviously. Thus, the temperature fluctuation amplitude was dominated by the primary factor between flow rate fluctuation amplitude and heat transfer coefficient.The experimental results show that the time average heat transfer coefficient of the narrow rectangular channel in rolling motion is lower than that under steady condition in laminar region, and the heat transfer is weakened by rolling motion. The heat transfer is weakened more deeply with the increase of rolling amplitude, rolling frequency and time average Reynolds number. However, the time average heat transfer coefficient of the narrow rectangular channel in rolling motion is higher than that under steady condition in transition region, and the heat transfer is enhanced by rolling motion. The degree of heat transfer enhanced increase first, reaches the maximum value and decrease after that. The heat transfer is enhanced more evidently with the increase of rolling amplitude and rolling frequency. The time average heat transfer coefficient of the narrow rectangular channel in rolling motion is the same as that under steady condition in turbulence region due to the lower amplitude of flow rate fluctuation.The instantaneous heat transfer coefficient under rolling motion conditions fluctuates periodically, the fluctuation period equals to the rolling period, and the variation tendency of the fluctuation amplitude is the same the trend of the flow rate fluctuation. A fitting correlation of the instantaneous heat transfer coefficient under rolling motion condition was deduced by manipulating the experimental data. The error of the calculations and the experimental results is less than ±25%.